Graphitization process

ABSTRACT

An improved process is provided for the production of a continuous length of a graphitic fibrous material through the catalysis of the graphitization reaction. While a continuous length of fibrous material capable of undergoing graphitization is continuously passed through a heating zone provided with an inert gaseous atmosphere having a maximum temperature of at least 2,000* C., at least one gaseous stream containing a catalytic quantity of a volatile alkyl borate in vapor form which is capable of catalyzing the graphitization reaction is introduced into the heating zone.

n'ited States Patent Ram [15] 3,6564 [4 Apr, 1%, 1172 [54]GRAPHITIZATION PROCESS [72] Inventor: Michael J. Ram, West Orange, NJ.[73] Assignee: Celanese Corporation, New York, NY. [22] Filed: June 10,1970 [2]] Appl. No.: 45,160

[52] US. Cl. ..23/209.1, 264/29, 252/502 [51] Int. Cl. ..C01b 31/07,C01b31/04 [58] Field of Search ..23/209.1, 209.2, 209.5;

[56] References Cited UNITED STATES PATENTS 8/1960 Jorgensen ..252/502Millington et al... ...23/209.1 X 3,449,007 6/1969 Stuetz ....23/209.13,552,923 l/1971 Carpenter et al. ..23/209.1

FOREIGN PATENTS OR APPLICATIONS 1,130,304 10/1968 Great Britain..23/209.1

OTHER PUBLICATIONS Allen et a1. Nature vol. 224 Nov. 15, 1969 pages 684-685 Klein Chemistry and Physics of Carbon vol. 2, 1966, pages 225- 227Primary Examiner-Edward J. Meros Attorney-Thomas J. Morgan, Charles B.Barris and Kenneth E. Macklin ABSTRACT 31 Claims, 1 Drawing lFigureGRAPHITIZATION PROCESS BACKGROUND OF THE INVENTION Processes involvingthe boron catalyzed conversion of amorphous carbon to graphitic carbonhave long been known. For instance, boron compounds such as boric acidhave been incorporated in a mass of graphitizable carbon and the samebaked to form massive graphite structures, such as graphite electrodes.Also, the pyrolytic codeposition of boron and carbon to formboron-containing pyrocarbons and boron pyrolitic graphite has beendisclosed.

In the search for high performance materials, considerable interest hasbeen focused upon graphitic fibrous materials. Graphite fibers aredefined herein as fibers which consist es sentially of carbon and have apredominant X-ray diffraction pattern characteristic of graphite.Amorphous carbon fibers or carbonized fibers, on the other hand, aredefined as fibers capable of undergoing graphitization in which the bulkof the fiber weight can be attributed to carbon and which exhibit anessentially amorphous X-ray diffraction pattern. Graphite fibersgenerally have a much higher modulus and a higher tenacity than doamorphous carbon fibers and in addition are more highly electrically andthermally conductive.

Industrial high performance materials of the future are projected tomake substantial utilization of fiber reinforced composites, andgraphite fibers theoretically have among the best properties of anyfiber for use as high strength reinforcement. Among these desirableproperties are corrosion and high temperature resistance, low density,high tensile strength, and most important, high modulus. Graphite is oneof the very few known materials whose tensile strength increases withtem perature. Uses for such graphite fiber reinforced composites includeaerospace structural components, rocket motor casings, deep-submergencevessels and ablative materials for heat shields on re-entry vehicles.

One of the major factors retarding the large-scale use of graphite fiberreinforced composites may be traced to the extreme costs commonlyrequired for the production of high modulus graphite fibers suitable foruse as reinforcement. Although the production of fibrous carbon bypyrolysis of hydrocarbon gases has been reported, this technique isgenerally not suitable for industrial. applications requiring goodquality control. Graphitization of amorphous carbon fibers derived fromfibrous organic precursors appears to be the only practical industrialroute available to form graphite fibers.

Many of the prior art methods for producing graphite fibers involve longprocessing periods, high power requirements, and/or expensive and bulkyheating apparatus, such as closed furnaces. For instance, when graphitetube furnaces are utilized the graphite tubes are of limited life andmust be periodically replaced. The processing and equipment costsrequired to produce graphite fibers commonly dwarf the fiber rawmaterial costs.

Amorphous carbon fibers have been; graphitized in the past by heatingfor extended periods of time, e.g., several hours, while present in aboron doped crucible. In an effort to expedite the graphitization of afibrous material, a technique has heretofore been proposed in which thefibrous material undergoing graphitization is first soaked in an aqueoussolution of boric acid, washed with water, and dried prior tographitization. While such a technique has proven to be effective incatalyzing the graphitization of the fibrous material, it has proven tobe unduly time consuming. For instance, it has proven to be essential towash the fiber following soaking in the solution of boric acid so thatan excessive quantity of the boron compound is not deposited upon thesurface of the fibrous precursor upon drying. It is essential also thatthe fibrous material be dried prior to heating at highly elevatedtemperatures in order to assure the attainment of adequate physicalproperties in the fibrous graphite product.

- In commonly assigned U.S. Ser. No. 45,161, filed on June 10, 1970 inthe name of Michael 1. Ram, is disclosed a graphitization process whichovercomes many of the disadvantages associated with prior art attemptsat the utilization of boron catalysis in the formation of graphitefibers. In such a process a continuous length of fibrous materialcapable of undergoing graphitization is continuously passed through aheating zone containing an inert gaseous atmosphere having a maximumtemperature of at least about 2,000 C. bounded by walls of graphiticcarbon in intimate association with a boron compound capable ofundergoing volatilization at a temperature below about 2,000 C. therebyenabling the volatilization of a catalytic quantity of boron capable ofcatalyzing graphitization within the heating zone.

It is an object of the invention to provide an improved graphitizationprocess for the production of graphitic fibrous materials.

It is an object of the invention to provide an improved graphitizationprocess which is capable of producing graphitic fibrous materials ofsuperior tensile properties.

It is an object of the invention to provide an improved graphitizationprocess which is capable of producing graphitic fibrous materialswithout sacrifice in tensile properties while reducing the maximumgraphitization temperature as well as the accompanying powerrequirements.

It is an object of the invention to provide an improved graphitizationprocess which is capable of producing graphitic fibrous materials whileoperating under conditions wherein the life of the apparatus utilized tocarry out the process is substantially lengthened.

It is another object of the invention to provide a catalyzedgraphitization process in which the quantity of catalytic compoundintroduced into the heating zone may be readily controlled and adjustedwith precision.

It is a further object of the invention to provide an improvedgraphitization process for the production of graphitic fibrous materialswhich is expeditiously carried out on a continuous basis.

These and other objects as well as the scope, nature, and utilization ofthe invention will be apparent from the following description andappended claims.

SUMMARY OF THE INVENTION It has been found that in a process for thegraphitization of a continuous length of a fibrous material capable ofundergoing graphitization comprising continuously passing the continuouslength of fibrous material through a heating zone containing an inertgaseous atmosphere having a maximum temperature of at least 2,000 C.until substantial graphitization occurs while preserving the originalfibrous configuration essentially intact, that improved results areachieved by introducing into the heating zone at least one gaseousstream containing a catalytic quantity of a volatile alkyl borate invapor form capable of catalyzing the graphitization of the fibrousmaterial within the heating zone.

In a preferred embodiment of the process the precursor is a stabilizedacrylic fibrous material and the heating zone is provided with atemperature gradient in which both carbonization and graphitization arecarried out.

DESCRIPTION OF THE DRAWING The drawing is a schematic illustration of arepresentative apparatus arrangement capable of carrying out the processof the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS yarn, tape, tow, strand, cable, orsimilar fibrous assemblage. In a preferred embodiment of the inventionthe fibrous material is a continuous multifilament yarn.

The fibrous material which is treated in the present process optionallymay be provided with a twist which tends to improve the handlingcharacteristics. For instance, a twist of about 0.1 to 5 tpi, andpreferably about 0.3 to 1.0 tpi may be imparted to a multifilament yarn.Also, a false twist may be used instead of or in addition to a realtwist. Alternatively, one may select continuous bundles of fibrousmaterial which possess essentially no twist.

The fibrous material which is graphitized in accordance with the presentprocess may be carbonaceous, contain at least about 90 per cent carbonby weight, and exhibit an essentially amorphous X-ray diffractionpattern. As is known in the art, amorphous carbon fibrous materialssuitable for graphitization may be formed by a variety of techniques.For instance, organic polymeric fibrous materials which are capable ofundergoing thermal stabilization may be initially stabilized bytreatment in an appropriate atmosphere at a moderate temperature (e.g.,200 to 400 C. and subsequently heated in an inert atmosphere at a morehighly elevated temperature, e.g., 900 to l,000 C., or more, until acarbonized fibrous material is formed which exhibits an essentiallyamorphous X-ray diffraction pattern. The exact temperature andatmosphere utilized during the initial stabilization of an organicpolymeric fibrous material commonly vary with the composition of theprecursor as will be apparent to those skilled in the art. During thecarbonization reaction elements present in the fibrous material otherthan carbon (e.g., oxygen and hydrogen) are substantially expelled.Suitable organic polymeric fibrous materials from which the fibrousmaterial capable of undergoing graphitization may be derived include anacrylic polymer, a cellulosic polymer, a polyamide, a polybenzimidazole,polyvinyl alcohol, etc. As discussed hereafter, acrylic polymericmaterials are particularly suited for use in the formation of thefibrous material capable of undergoing graphitization which is employedin the present process. Illustrative examples of suitable cellulosicmaterials include the natural and regenerated forms of cellulose, e.g.,rayon. Illustrative examples of suitable polyamide materials include thearomatic polyamides, such as nylon 6T, which is formed by thecondensation of hexamethylenediamine and terephthalic acid. Anillustrative example of a suitable polybenzimidazole is poly-2,2'-m-phenylene-5,5-bibenzimidazole.

A fibrous acrylic polymeric material prior to stabilization may beformed primarily of recurring acrylonitrile units. For instance, theacrylic polymer should contain not less than about 85 mol per cent ofrecurring acrylonitrile units with not more than about 15 mol per centof a monovinyl compound which is copolymerizable with acrylonitrile suchas styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinylchloride, vinylidene chloride, vinyl pyridine, and the like, or aplurality of such monovinyl compounds.

During the formation of a preferred carbonized starting material for usein the present process multifilament bundles of an acrylic fibrousmaterial may be initially stabilized in an oxygen-containing atmosphere(i.e., preoxidized) on a continuous basis in accordance with theteachings of U.S. Ser. No. 749,957, filed Aug. 5, 1968, of Dagobert E.Stuetz, which is assigned to the same assignee as the instant inventionand is herein incorporated by reference. More specifically, the acrylicfibrous material should be either an acrylonitrile homopolymer or anacrylonitrile copolymer which contains no more than about 5 mol per centof one or more monovinyl comonomers copolymerized with acrylonitrile. Ina particularly preferred embodiment of the invention the fibrousmaterial is derived from an acrylonitrile homopolymer. The stabilizedacrylic fibrous material which is preoxidized in an oxygen-containingatmosphere is black in appearance, contains a bound oxygen content of atleast 7 per cent by weight as determined by the Unterzaucher analysis,retains its original fibrous configuration essentially intact, and isnonburning when subjected to an ordinary match flame.

In the present process a continuous length of the fibrous materialcapable of undergoing graphitization is continuously passed through aheating zone having a maximum temperature of at least 2,000, e.g., amaximum temperature of 2,000 to 3,100 C. (preferably 2,400 to 3,100 C.),containing an inert gaseous atmosphere for a residence time sufficientto substantially convert the fibrous material to graphitic carbon whileretaining its original fibrous configuration essentially intact.Suitable inert gaseous atmospheres for the heating zone includenitrogen, argon, helium, etc. For instance, a continuous length of anamorphous carbon fibrous material, e.g., a multifilament yarn may bepassed through the heating zone while at a graphitization temperature ofat least 2,000 C. for a residence time of about 5 seconds to 4 minutesto produce graphitization. Longer graphitization heating times may beselected but generally yield no commensurate advantage. Preferredresidence times while within about 50 C. of the maximum graphitizationtemperature commonly range from about 10 seconds to 200 seconds.

In a preferred embodiment of the process a continuous length of astabilized acrylic fibrous material which is nonbuming when subjected toan ordinary match fiame and derived from an acrylic fibrous materialselected from the group consisting of an acrylonitrile homopolymer andacrylonitrile copolymers which contain at least about mol per cent ofacrylonitrile units and up to about 15 mol per cent of one or moremonovinyl units copolymerized therewith is continuously passed through aheating zone containing an inert gaseous atmosphere and a temperaturegradient in which said fibrous material is initially carbonized, and inwhich said carbonized fibrous material is heated to a maximumtemperature of at least 2,000 C. until substantial graphitizationoccurs. Representative inert gaseous atmospheres for the heating zone inwhich both carbonization and graphitization are accomplished includenitrogen, argon, helium, etc.

When the fibrous material supplied to the heating zone is a stabilizedacrylic fibrous material it may be carbonized and graphitized whilepassing through a temperature gradient in accordance with the proceduresdescribed in commonly assigned U.S. Ser. Nos. 777,275, filed Nov. 20,1968 of Charles M. Clarke entitled Process for the ContinuousCarbonization of a Stabilized Acrylic Fibrous Material; 17,780 filedMar. 9, 1970 ofCharles M. Clarke, Michael .1. Ram, and John P. Riggsentitled Improved Process for the Carbonization of a Stabilized AcrylicFibrous Material," and 17,832 filed Mar. 9, 1970 of Charles M. Clarke,Michael J. Ram, and Arnold .I. Rosenthal entitled Production of HighTenacity Graphitic Fibrous Materials. Each of these disclosures isherein incorporated by reference.

In accordance with a particularly preferred embodiment of the process acontinuous length of stabilized acrylic fibrous material which isnon-buming when subjected to an ordinary match flame and derived from anacrylic fibrous material selected from the group consisting of anacrylonitrile homopolymer and acrylonitrile copolymers which contain atleast about 85 mol per cent of acrylonitrile units and up to about 15mol per cent of one or more monovinyl units copolymerized therewith isconverted to a graphitic fibrous material while preserving the originalfibrous configuration essentially intact while passing through acarbonization/graphitization heating zone containing an inert gaseousatmosphere and a temperature gradient in which said fibrous material israised within a period of about 20 to about 300 seconds from about 800C. to a temperature of about l,600 C. to form a continuous length ofcarbonized fibrous material, and in which said carbonized fibrousmaterial is subsequently raised from about l,600 C. to a maximumtemperature of at least about 2,400 C. within a period of about 3 to 300seconds where it is maintained for about 10 seconds to about 200 secondsto form a continuous length of graphitic fibrous material.

The equipment utilized to produce the heating zone used to producegraphitization or carbonization and graphitization in the process of thepresent invention may be varied as will be apparent to those skilled inthe art. It is essential that the apparatus selected be capable ofproducing the required temperature while excluding the presence of anoxidizing atmosphere.

In a preferred embodiment of the invention, the continuous length offibrous material undergoing graphitization or carbonization andgraphitization is heated by use of an induction furnace. In such aprocedure the fibrous material may be passed in the direction of itslength through a hollow graphite tube or other susceptor which issituated within the windings of the induction coil. By varying thelength of the graphite tube, the length of the induction coil, and therate at which the fibrous material is passed through the graphite tube,many apparatus arrangements capable of carrying out the graphitizationor carbonization and graphitization may be selected. For large-scaleproduction, it is of course preferred that relatively long tubes orsusceptors be used so that the fibrous material may be passed throughthe same at a more rapid rate while being graphitized or carbonized andgraphitized. The temperature gradient of a given apparatus may bedetemiined by conventional optical pyrometer measurements as will beapparent to those skilled in the art. The fibrous material because ofits small mass and relatively large surface area instantaneously assumesessentially the same temperature as the inert gaseous atmosphere of theheating zone through which it is continuously passed.

During the formation of graphitic carbon within the continuous length offibrous material a tensional force may be optionally applied to thebundles undergoing graphitization in order to provide efficient handlingof the fibrous material and/or to modify the physical properties of thesame.

During the graphitization reaction the inert gaseous atmosphere iscommonly caused to flow through the heating zone while preserving therequisite heating. For instance, when an induction furnace is employed,the inert gaseous atmosphere may be continuously introduced through oneor more small apertures provided in the walls of a hollow graphite tubewhich is surrounded by an induction coil. The inert gaseous atmosphereaccordingly exits from the graphite tube through the ends thereof. Theflow of the inert gaseous atmosphere out of each end of the graphitetube accordingly substantially excludes the introduction of air or anoxidizing atmosphere within the heating zone.

At least one gaseous stream containing a catalytic quantity of avolatile alkyl borate in vapor form capable of catalyzing thegraphitization of the fibrous material is preferably introduced into theheating zone so that the vapor stream directly impinges upon thecontinuous length of fibrous material, or at least is provided in theinert gaseous atmosphere of the heating zone immediately adjacent thefibrous material. Preferably the point of introduction of the streamcontaining the volatile alkyl borate in gaseous form is identical tothat location where the fibrous material enters the heating zone or isin relatively close proximity thereto. Preferably the gas stream isintroduced into the heating zone in close proximity to the fibrousmaterial prior to raising the temperature of the fibrous material aboveabout 500 C. For this reason the heating zone is preferably providedwith a temperature gradient in which the fibrous material isprogressively elevated to a maximum temperature of at least 2,000 C.where substantial graphitization is accomplished. If desired, anauxiliary graphite tube or susceptor may be provided in series with themain graphite tube of the induction furnace to extend the length of theentrance portion of the heating zone, and the stream of boron compoundas well as the starting fibrous material introduced into such extendedportion. It has been found that if the stream of boron compound in vaporform is introduced solely into that portion of the heating zone which isat a highly elevated temperature (i.e., at least 2,000 C.), then thereis a tendency for the boron compound to undergo a prompt reaction withthe walls of the heating zone and thereby to become at least partiallyunavailable in the catalysis of the graphitization of the fibrousmaterial.

In a preferred embodiment of the process the continuous length offibrous material is supplied to the heating zone in an essentiallyanhydrous form which is absorbent with respect to the volatile alkylborate which contacts the same within the heating zone. If desired thestream of volatile alkyl borate in vapor form may be at least partiallydiluted with an inert gas, such as nitrogen, argon, or helium, which ispreferably identical to the inert gaseous atmosphere otherwise suppliedto the heating zone.

The volatile alkyl borate capable of catalyzing the graphitizationreaction is preferably introduced into the inert gaseous atmospheresurrounding the continuous length of fibrous material in at least aportion of the heating zone in a concentration of about 200 to 20,000parts per million by volume. In a particularly preferred embodiment ofthe process the volatile alkyl borate is introduced into the gaseousatmosphere surrounding the continuous length of fibrous material in atleast a portion of the heating zone in a concentration of about 200 to2,000 parts per million by volume. The quantity of the volatile alkylborate introduced into the heating zone is preferably about 0.01 to 4per cent by weight based upon the weight of fibrous material introducedinto the heating zone. In a particularly preferred embodiment of theinvention the quantity of volatile alkyl borate introduced into theheating zone is about 0.04 to 0.4 per cent by weight based upon theweight of the fibrous material introduced into the heating zone.

It is preferred that the alkyl borate possess substantial volatility at500 C. or below and is conveyed to the heating zone while at atemperature not in excess of 500 C. In a particularly preferredembodiment of the process the alkyl borate possesses substantialvolatility at about room temperature (i.e., about 25 C.) therebyfacilitating convenient handling and introduction of the same withoutresorting to elevated temperatures.

Any alkyl borate which is capable of being introduced into the heatingzone in vapor form may be selected for use in the process. The preferredvolatile alkyl borates for use in the process are the alkyl borates ofthe formula B(OR) where R is an alkyl group having 1 to 5 carbon atoms.Such alkyl borates include: trimethyl borate, B(OCI-I sometimesidentified as methyl borate, or trimethoxyborine; triethyl borate, B(OCH.-.)=, tripropyl borate, B (QC;,H triisopropyl borate. B [O(CH Cl-I 1tributyl borate, B(OC H and triamyl borate, B(OC l-l The particularlypreferred alkyl borate for use in the process is trimethyl borate.

Other representative alkyl borates possessing sufficient volatility foruse in the present process include higher molecular weight boric acidesters such as tricyclohexyl borate, tridodecyl borate, B(OC H- I andtri hexylene glycolbiborate, B tO C H l The introduction into theheating zone of a gaseous stream containing a volatile alkyl borate asdefined herein enables the graphitization of the fibrous material toproceed in a more efficient manner. Graphitic fibrous materialsexhibiting improved tensile properties, Youngs modulus as well astensile strength, may be formed on a continuous basis in accordance withthe present process without modification of the graphitization heatingprofile. Alternatively, through the use of the present process it ispossible to decrease the maximum temperature experienced by the fibrousmaterial within the heating zone while still achieving highly acceptabletensile properties within the resulting product.

The ability for one to operate at a lower maximum graphitizationtemperature offers a substantial cost reduction since less power isrequired and the usable life of the apparatus utilized in the process isextended. For instance, the graphite tube or susceptor of an inductionfurnace may have its life extended many times (e.g., five to 10 times)by simply lowering the maximum graphitization temperature from about2,900 C. to 2,700 C. Not only is one spared the cost of a replacementgraphite tube, but down time is eliminated which would otherwise beconsumed while replacing the graphite tube, starting up the furnace, andallowing it to again come to equilibrium conditions.

No immersing, rinsing, and drying of the starting fibrous material isrequired as has been practiced in a prior art boron catalysis techniquein which the starting material is initially soaked in a solution ofboric acid. Through the use of the present process the catalyticquantity of the boron compound introduced into the heating zone isreadily controlled and subject to accurate adjustment throughout theduration of the graphitization process.

The following examples are given as specific illustrations of theinvention. It should be understood, however, that the invention is notlimited to the specific details set forth in the examples. Reference ismade in the examples to the drawing.

EXAMPLE I A continuous length of 1,600 fil unwashed dry spunacrylonitrile homopolymer continuous filament yarn having a total denierof 2,000 was selected as the starting material. The yarn was orientedand drawn to a single filament tenacity of about 3.2 grams per denier.The yarn was subjected to a healing treatment in which residualN,N-dimethyl formamide spinning solvent was evolved by passage for 6minutes through a muffle furnace provided with air at 185 C. duringwhich time the yarn shrank 10 per cent in length. The yarn wascontinuously stabilized in accordance with the teachings of U.S. Ser.No. 749,957, filed Aug. 5, I968, of Dagobert E. Stuetz, which isassigned to the same assignee as the present invention and is hereinincorporated by reference. During the stabilization reaction (i.e.,preoxidation) the yarn was continuously passed in the direction of itslength in the absence of shrinkage through a multi-wrap skewed roll ovenprovided with an air atmosphere at 270 C. for a residence time of I47minutes. The resulting stabilized yarn was black in appearance,containeda bound oxygen content of 10.85 per cent by weight as determined by theUnterzaucher analysis, and was non-burning when subjected to an ordinarymatch flame.

The preoxidized yarn was stored in a forced air oven at 1 l C. whilewound upon a bobbin following stabilization. The yarn was next unwoundfrom the bobbin and passed through a drying zone (not shown) whereinvolatiles were substantially removed prior to introduction into theheating zone wherein carbonization and graphitization were accomplished.The drying zone consisted of one 12 inch muffle furnace provided withcirculating air at 200 C.

The dried stabilized yarn l was next continuously passed at a rate ofabout 1.5 inches/minute in the direction of its length through a Lepel450 KC induction furnace 2 utilizing a 20 KW power source wherein bothcarbonization and graphitization were accomplished. The inductionfurnace comprised a 10 turn water cooled copper coil 4 having an innerdiameter of three-fourths and a length of 2 inches and a hollow graphitetube or susceptor 6 suspended within the coil having a length of 8 /2inches, an outer diameter of one-half inch and an inner diameter ofone-eighth inch through which the yarn was continuously passed. Four Mainch holes were provided in the wall of graphite tube 6. The hollowgraphite tube 6 was held in position by supports 7. The copper coil 4which encompassed a portion of the hollow graphite tube 6 was positionedat a location essentially equidistant from the respective ends of thegraphite tube. The copper coil 4 was connected to 20 KW power source 8.An auxiliary graphite tube 10 through which the fibrous material passedwas placed at the entrance end of graphite tube 6. The auxiliary tube 10had an overall length of 5 3/4 inches, one-half inch of whichencompassed the end of graphite tube 6. The outer diameter of theauxiliary tube 10 was 1 inch and the inner diameter of the auxiliarytube was one-fourth inch. A stainless steel enclosure or housing 12having a wall thickness of about one-fourth inch and an overall lengthof about 11 inches, a height of about 6 inches, and a width of about 6inches surrounded the induction furnace 2. The resulting graphite yarn14 as it left graphite tube 6 passed through cylindrical orifice 16formed of graphite having an outer diameter of one-half inch and aninner diameter of oneeighth inch.

Nitrogen was passed through line 18 to rotometer 20 which delivered thenitrogen at a rate of 25 standard cubic feet per hour through line 22.Line 22 branched into line 24 which communicated with a centrallylocated orifice in the wall of housing 12, and line 26 whichcommunicated with rotometer 28. A stream of nitrogen from line 24entered the interior of the housing 12 at a rate of 24.976 standardcubic feet per hour. Nitrogen was delivered from rotometer 28 throughline 30 at a flow rate of 0.024 standard cubic feet per hour. The streamof nitrogen from line 30 was bubbled through liquid trimethyl borate 32present in vessel 34. Nitrogen gas as well as trimethyl borate in vaporform was withdrawn from the space 36 above the liquid trimethyl borate32 through line 38. An ice bath 40 surrounded vessel 34 to insure aconstant vapor pressure for the trimethyl borate. Approximately 10 percent by volume of the gaseous stream in line 38 was trimethyl borate,and approximately per cent by volume of the gaseous stream in line 38was nitrogen. The gaseous stream containing the trimethyl borate invapor form is introduced into the interior of auxiliary graphite tube 10through an orifice in the wall thereof located 4% inches from theentrance end thereof.

The nitrogen gas which entered housing 12 through line 24 entered thegraphite tube 6 through holes 5 as well as the open end 42 thereof.Approximately one-half of the gaseous atmosphere present within housing12 exits through orifice l6 and approximately one-half of the gaseousatmosphere exits through the exposed end 44 of auxiliary graphite tube10.

3.9 X 10' grams of trimethyl borate in vapor form entered graphite tube10 via line 38 per minute and directly impinged upon the yarn. The yarnl was at about C. when it entered the exposed end of graphite tube 10.The temperature of the yarn within auxiliary tube 10 at the point wherethe vapor stream of trimethyl borate impinged upon the same was about350 C. 0.0085 grams of yarn passed the point where the vapor stream oftrimethyl borate impinged upon the same per minute. Accordingly a streamof trimethyl borate in vapor form was supplied to the inert gaseousatmosphere of the heating zone in a quantity of about 0.045 per cent byweight based upon the weight of the yarn. The quantity of trimethylborate in the inert atmosphere surrounding the yarn in the immediatearea of the heating zone wherein the boron compound was introduced wasabout 200 parts per million.

While passing through the heating zone defined by the auxiliary graphitetube 10 and graphite tube 6, the yarn was raised from about 100 C. to atemperature of 800 C. in approximately 300 seconds, from 800 C. to l,600C. in approximately 60 seconds to produce a carbonized yarn, and from1,600 C. to a maximum temperature of approximately 2,700 C. inapproximately 40 seconds where it was maintained 50 C. for approximately40 seconds. While passing through the heating zone constant longitudinaltensions of 300, 400, and 500 grams were maintained upon portions of theyarn at various points in time. The resulting yarn 14 exhibited agraphitic carbon X-ray diffraction pattern and a specific gravity ofabout 2.0. The following single filament tensile properties wereobtained for the various graphite yarn samples.

Tension in Grams Tensile Strength Young's Modulus 300 323x10 psi 87x10psi 500 400 l0 si 97x10 psi EXAMPLE 11 Example I was repeated with theexceptions indicated.

A stream of nitrogen gas from line 24 entered the interior of housing 12at a rate of 24.76 standard cubic feet per hour. Nitrogen was deliveredfrom rotometer 28 through line 30 at a rate of 0.24 standard cubic feetper hour. 3.9 X 10 grams of trimethyl borate in vapor fonn enteredgraphite tube 10 via line 38 per minute and directly impinged upon theyarn. Trimethyl borate in vapor form was supplied to the inert gaseousatmosphere of the heating zone in a quantity of about 0.45 per cent byweight based upon the weight of the yarn. The quantity of trimethylborate in the inert atmosphere surrounding the yarn in the immediatearea of the heating zone wherein the boron compound was introduced wasabout 2,000 parts per million.

The following single filament tensile properties were obtained for thevarious graphite yarn samples.

Tension in Grams Tensile Strength Young's Modulus 300 330x10 psi 82X lpsi 400 asoxio" psi 104x10 psi 500 430Xl0 psi 110x10 psi In acomparative graphitization procedure Examples I and II were repeated inan identical apparatus with the exception that no boron compound wasintroduced into the heating zone. A stream of nitrogen gas from line 24entered the interior of housing 12 at a rate of 25 standard cubic feetper hour. The overall tensile properties of the resulting graphite yarnwere generally lower than those obtained in Examples I and II. Thefollowing single filament tensile properties were determined for thevarious graphite yarn samples.

Tension in Grams Tensile Strength Young's Modulus 300 328x psi 80x10 psi40o 373x10 psi 90x10 psi 500 455x10 psi 93X 10 psi EXAMPLE III Example Iwas repeated with the exceptions indicated.

While passing through the heating zone defined by the auxiliary graphitetube 10 and graphite tube 6, the yarn was raised from about 100 C. to atemperature of 800 C. in approximately 300 seconds, from 800 C. to l,600C. in approximately 60 seconds to produce a carbonized yarn, and from1,600" C. to a maximum temperature of approximately 2,900 C. inapproximately 40 seconds where it was maintained fl0 C. forapproximately 40 seconds. The following single filament tensileproperties were determined for the various graphite yarn samples.

Tension in Grams Tensile Strength Young's Modulus 300 430x10" psi 108x10psi 400 480x10" psi lllXlO psi 500 330x10 psi l04 10 psi It will benoted that the tensile properties were generally improved when employinga higher maximum graphitization temperature.

In Examples I, II, and III no boron carbide was detected in theresulting graphite yarn by conventional X-ray diffraction studies.

In a comparative graphitization procedure Example III was repeated in anidentical apparatus with the exception that no boron compound wasintroduced into the heating zone. A stream of nitrogen gas from line 24entered the interior of housing 12 at a rate of 25 standard cubic feetper hour. The overall tensile properties of the resulting graphite yarnwere generally lower than those obtained in Example III. The followingsingle filament tensile properties were determined for the variousgraphite yarn samples.

Tension in Grams Tensile Strength Young's Modulus 300 323x10 psi 94x10"psi 400 30BX10 psi 87x10 psi 500 465x10 psi 104x10 psi It will be notedthat the tensile properties achieved in the above comparativegraphitization run are generally comparable to those achieved inExamples I and II in accordance with the present invention whenoperating at a lower maximum graphitization temperature. By operating ata lower maximum graphitization temperature such as that employed inExamples I and H the useful life of graphite tube 6 is substantiallyincreased.

EXAMPLE IV Example I is repeated with the following exceptions.

The continuous length of continuous filament yarn which is introducedinto the heating zone is a carbonaceous yarn derived from anacrylonitrile homopolymer containing about 99 per cent carbon by weightand exhibits an essentially amorphous X-ray diffraction pattern.Substantially similar results are achieved.

Although the invention has been described with preferred embodiments, itis to be understood that variations and modifications may be resorted toas will be apparent to those skilled in the art. Such variations are tobe considered within the purview and scope of the claims appendedhereto.

I claim:

1. In a process for the graphitization of a continuous length of afibrous material capable of undergoing graphitization comprisingcontinuously passing said continuous length of fibrous material througha heating zone containing an inert gaseous atmosphere having a maximumtemperature of at least 2,000 C. until substantial graphitization occurswhile preserving the original fibrous configuration essentially intact;the improvement of introducing into said heating zone at least onegaseous stream containing a catalytic quantity of a volatile alkylborate in vapor form capable of catalyzing said graphitization of saidfibrous material within said heating zone.

2. A process according to claim 1 wherein said continuous length offibrous material capable of undergoing graphitization is a carbonaceousfibrous material containing at least about 90 per cent carbon by weightand having an essentially amorphous X-ray diffraction pattern.

3. A process according to claim 1 wherein said continuous length offibrous material is a continuous multifilament yarn.

4. A process according to claim 1 wherein said inert gaseous atmosphereis selected from the group consisting of nitrogen, argon, and helium.

5. A process according to claim 1 wherein said heating zone contains aninert gaseous atmosphere having a maximum temperature of about 2,400 to3,l00 C.

6. A process according to claim 1 wherein said volatile alkyl borate isan alkyl borate of the formula B(OR) where R is an alkyl group havingone to five carbon atoms.

7. A process according to claim 1 wherein said volatile alkyl borate istrimethyl borate.

8. A process according to claim 1 wherein said volatile alkyl borate invapor form is introduced into said inert gaseous atmosphere surroundingsaid continuous length of fibrous material in a concentration of about200 to.20,000 parts per million prior to heating said fibrous materialabove about 500 C. in said heating zone.

9. A process according to claim 7 wherein said trimethyl borate in vaporform is introduced into said inert gaseous atmosphere surrounding saidcontinuous length of fibrous material in a concentration of about 200 to2,000 parts per million.

10. In a process for converting a stabilized acrylic fibrous materialwhich is non-burning when subjected to an ordinary match flame andderived from an acrylic fibrous material selected from the groupconsisting of an acrylonitrile homopolymer and acrylonitrile copolymerswhich contain at least about mol per cent of acrylonitrile units and upto about 15 mol per cent of one or more monovinyl units copolymerizedtherewith to a graphitic fibrous material while preserving the originalfibrous configuration essentially intact comprising continuously passinga continuous length of said fibrous material through a heating zonecontaining an inert gaseous atmosphere and a temperature gradient inwhich said fibrous material is initially carbonized, and in which saidcarbonized fibrous material is heated to a maximum temperature of atleast 2,000 C. until substantial graphitization occurs; the improvementof introducing into said heating zone at least one gaseous streamcontaining a catalytic quantity of volatile alkyl borate in vapor formcapable of catalyzing said graphitization of said fibrous materialwithin said heating zone.

11. A process according to claim 10 wherein said stabilized acrylicfibrous material exhibits a bound oxygen content of at least about 7 percent by weight.

12. A process according to claim 10 wherein said stabilized acrylicfibrous material is derived from an acrylonitrile homopolymer.

13. A process according to claim 10 wherein said stabilized acrylicfibrous material is derived from an acrylonitrile copolymer whichcontains at least about 95 mol per cent of acrylonitrile units and up toabout mol per cent of one or more monovinyl units copolymerizedtherewith.

14. A process according to claim wherein said continuous length ofstabilized acrylic fibrous material is a continuous multifilament yarn.

15. A process according to claim 10 wherein said inert gaseousatmosphere is selected from the group consisting of nitrogen, argon, andhelium.

16. A process according to claim 10 wherein said carbonized fibrousmaterial is heated in said inert gaseous atmosphere to a maximumtemperature of about 2,400 to 3,l00 C. until substantial graphitizationoccurs.

17. A process according to claim 10 wherein said volatile alkyl borateis an alkyl borate of the formula B (OR) where R is an alkyl grouphaving one to five carbon atoms.

18. A process according to claim 10 wherein said volatile alkyl borateis trimethyl borate.

19. A process according to claim 10 wherein said volatile alkyl boratein vapor form is introduced into said inert gaseous atmospheresurrounding said continuous length of fibrous material in aconcentration of about 200 to 20,000 parts per million, and in aquantity of about 0.01 to 4 per cent by weight based upon the weight ofthe stabilized acrylic fibrous material introduced into said heatingzone.

20. A process according to claim 19 wherein said volatile alkyl borateis trimethyl borate and said trimethyl borate is introduced into saidinert gaseous atmosphere surrounding said continuous length ofstabilized acrylic fibrous material in a concentration of about 200 to2,000 parts per million prior to heating said fibrous material aboveabout 500 C. in said heating zone.

21. In a process for converting a stabilized acrylic fibrous materialwhich is non-burning when subjected to an ordinary match flame andderived from an acrylic fibrous material selected from the groupconsisting of an acrylonitrile homopolymer and acrylonitrile copolymerswhich contain at least about 85 mol per cent of acrylonitrile units andup to about mol per cent of one or more monovinyl units copolymerizedtherewith to a graphitic fibrous material while preserving the originalfibrous configuration essentially intact comprising continuously passinga continuous length of said fibrous material through a heating zonecontaining an inert gaseous atmosphere and a temperature gradient inwhich said fibrous material is raised within a period of about 20 toabout 300 seconds from about 800 C. to a temperature of about l,600 C.to form a continuous length of carbonized fibrous material, and in whichsaid carbonized fibrous material is subsequently raised from about 1,600C. to a maximum temperature of at least about 2,400 C. within a periodof about 3 to 300 seconds where it is maintained for about 10 seconds toabout 200 seconds to form a continuous length of graphitic fibrousmaterial; the improvement of introducing into said heating zone at leastone gaseous stream containing a catalytic quantity of a volatile alkylborate in vapor form capable of catalyzing said graphitization of saidfibrous material within said heating zone.

22. A process according to claim 21 wherein said stabilized acrylicfibrous material exhibits a bound oxygen content of at least about 7 percent b weight.

23. A process accor ing to claim 21 wherein said stabilized acrylicfibrous material is derived from an acrylonitrile homopolymer.

24. A process according to claim 21 wherein said stabilized acrylicfibrous material is derived from an acrylonitrile copolymer whichcontains at least about mol per cent of acrylonitrile units and up toabout 5 mol per cent of one or more monovinyl units copolymerizedtherewith.

25. A process according to claim 21 wherein said continuous length ofstabilized acrylic fibrous material is a continuous multifilament yarn.

26. A process according to claim 21 wherein said inert gaseousatmosphere is selected from the group consisting of nitrogen, argon, andhelium.

27. A process according to claim 21 wherein said carbonized fibrousmaterial is heated in said inert gaseous atmosphere to a maximumtemperature of about 2,400 to 3, 1 00 C. to form a continuous length ofgraphitic fibrous material.

28. A process according to claim 21 wherein said volatile alkyl borateis an alkyl borate of formula B(OR) where R is an alkyl group having oneto five carbon atoms.

29. A process according to claim 21 wherein said volatile alkyl borateis trimethyl borate.

30. A process according to claim 21 wherein said volatile alkyl boratein vapor form is introduced into said inert gaseous atmospheresurrounding said continuous length of fibrous material in aconcentration of about 200 to 20,000 parts per million, and in aquantity of about 0.04 to 4 per cent by weight based upon the weight ofthe stabilized acrylic fibrous material introduced into said heatingzone.

31. A process according to claim 30 wherein said volatile alkyl borateis trimethyl borate and said trimethyl borate is introduced into saidinert gaseous atmosphere surrounding said continuous length ofstabilized acrylic fibrous material in a concentration of about 200 to2,000 parts per million prior to heating said fibrous material aboveabout 500 C. in said heating zone.

2. A process according to claim 1 wherein said continuous length offibrous material capable of undergoing graphitization is a carbonaceousfibrous material containing at least about 90 per cent carbon by weightand having an essentially amorphous X-ray diffraction pattern.
 3. Aprocess according to claim 1 wherein said continuous length of fibrousmaterial is a continuous multifilament yarn.
 4. A process according toclaim 1 wherein said inert gaseous atmosphere is selected from the groupconsisting of nitrogen, argon, and helium.
 5. A process according toclaim 1 wherein said heating zone contains an inert gaseous atmospherehaving a maximum temperature of about 2,400* to 3,100* C.
 6. A processaccording to claim 1 wherein said volatile alkyl borate is an alkylborate of the formula B(OR)3 where R is an alkyl group having one tofive carbon atoms.
 7. A process according to claim 1 wherein saidvolatile alkyl borate is trimethyl borate.
 8. A process according toclaim 1 wherein said volatile alkyl borate in vapor form is introducedinto said inert gaseous atmosphere surrounding said continuous length offibrous material in a concentration of about 200 to 20,000 parts permillion prior to heating said fibrous material above about 500* C. insaid heating zone.
 9. A process according to claim 7 wherein saidtrimethyl borate in vapor form is introduced into said inert gaseousatmosphere surrounding said continuous length of fibrous material in aconcentration of about 200 to 2,000 parts per million.
 10. In a processfor converting a stabilized acrylic fibrous material which isnon-burning when subjected to an ordinary match flame and derived froman acrylic fibrous material selected from the group consisting of anacrylonitrile homopolymer and acrylonitrile copolymers which contain atleast about 85 mol per cent of acrylonitrile units and up to about 15mol per cent of one or more monovinyl units copolymerized therewith to agraphitic fibrous material while preserving the original fibrousconfiguration essentially intact comprising continuously passing acontinuous length of said fibrous material through a heating zonecontaining an inert gaseous atmosphere and a temperature gradient inwhich said fibrouS material is initially carbonized, and in which saidcarbonized fibrous material is heated to a maximum temperature of atleast 2,000* C. until substantial graphitization occurs; the improvementof introducing into said heating zone at least one gaseous streamcontaining a catalytic quantity of volatile alkyl borate in vapor formcapable of catalyzing said graphitization of said fibrous materialwithin said heating zone.
 11. A process according to claim 10 whereinsaid stabilized acrylic fibrous material exhibits a bound oxygen contentof at least about 7 per cent by weight.
 12. A process according to claim10 wherein said stabilized acrylic fibrous material is derived from anacrylonitrile homopolymer.
 13. A process according to claim 10 whereinsaid stabilized acrylic fibrous material is derived from anacrylonitrile copolymer which contains at least about 95 mol per cent ofacrylonitrile units and up to about 5 mol per cent of one or moremonovinyl units copolymerized therewith.
 14. A process according toclaim 10 wherein said continuous length of stabilized acrylic fibrousmaterial is a continuous multifilament yarn.
 15. A process according toclaim 10 wherein said inert gaseous atmosphere is selected from thegroup consisting of nitrogen, argon, and helium.
 16. A process accordingto claim 10 wherein said carbonized fibrous material is heated in saidinert gaseous atmosphere to a maximum temperature of about 2,400* to3,100* C. until substantial graphitization occurs.
 17. A processaccording to claim 10 wherein said volatile alkyl borate is an alkylborate of the formula B (OR)3 where R is an alkyl group having one tofive carbon atoms.
 18. A process according to claim 10 wherein saidvolatile alkyl borate is trimethyl borate.
 19. A process according toclaim 10 wherein said volatile alkyl borate in vapor form is introducedinto said inert gaseous atmosphere surrounding said continuous length offibrous material in a concentration of about 200 to 20,000 parts permillion, and in a quantity of about 0.01 to 4 per cent by weight basedupon the weight of the stabilized acrylic fibrous material introducedinto said heating zone.
 20. A process according to claim 19 wherein saidvolatile alkyl borate is trimethyl borate and said trimethyl borate isintroduced into said inert gaseous atmosphere surrounding saidcontinuous length of stabilized acrylic fibrous material in aconcentration of about 200 to 2,000 parts per million prior to heatingsaid fibrous material above about 500* C. in said heating zone.
 21. In aprocess for converting a stabilized acrylic fibrous material which isnon-burning when subjected to an ordinary match flame and derived froman acrylic fibrous material selected from the group consisting of anacrylonitrile homopolymer and acrylonitrile copolymers which contain atleast about 85 mol per cent of acrylonitrile units and up to about 15mol per cent of one or more monovinyl units copolymerized therewith to agraphitic fibrous material while preserving the original fibrousconfiguration essentially intact comprising continuously passing acontinuous length of said fibrous material through a heating zonecontaining an inert gaseous atmosphere and a temperature gradient inwhich said fibrous material is raised within a period of about 20 toabout 300 seconds from about 800* C. to a temperature of about 1,600* C.to form a continuous length of carbonized fibrous material, and in whichsaid carbonized fibrous material is subsequently raised from about1,600* C. to a maximum temperature of at least about 2,400* C. within aperiod of about 3 to 300 seconds where it is maintained for about 10seconds to about 200 seconds to form a continuous length of graphiticfibrous material; the improvement of introducing into said heatinG zoneat least one gaseous stream containing a catalytic quantity of avolatile alkyl borate in vapor form capable of catalyzing saidgraphitization of said fibrous material within said heating zone.
 22. Aprocess according to claim 21 wherein said stabilized acrylic fibrousmaterial exhibits a bound oxygen content of at least about 7 per cent byweight.
 23. A process according to claim 21 wherein said stabilizedacrylic fibrous material is derived from an acrylonitrile homopolymer.24. A process according to claim 21 wherein said stabilized acrylicfibrous material is derived from an acrylonitrile copolymer whichcontains at least about 95 mol per cent of acrylonitrile units and up toabout 5 mol per cent of one or more monovinyl units copolymerizedtherewith.
 25. A process according to claim 21 wherein said continuouslength of stabilized acrylic fibrous material is a continuousmultifilament yarn.
 26. A process according to claim 21 wherein saidinert gaseous atmosphere is selected from the group consisting ofnitrogen, argon, and helium.
 27. A process according to claim 21 whereinsaid carbonized fibrous material is heated in said inert gaseousatmosphere to a maximum temperature of about 2,400 to 3,100* C. to forma continuous length of graphitic fibrous material.
 28. A processaccording to claim 21 wherein said volatile alkyl borate is an alkylborate of formula B(OR)3 where R is an alkyl group having one to fivecarbon atoms.
 29. A process according to claim 21 wherein said volatilealkyl borate is trimethyl borate.
 30. A process according to claim 21wherein said volatile alkyl borate in vapor form is introduced into saidinert gaseous atmosphere surrounding said continuous length of fibrousmaterial in a concentration of about 200 to 20,000 parts per million,and in a quantity of about 0.04 to 4 per cent by weight based upon theweight of the stabilized acrylic fibrous material introduced into saidheating zone.
 31. A process according to claim 30 wherein said volatilealkyl borate is trimethyl borate and said trimethyl borate is introducedinto said inert gaseous atmosphere surrounding said continuous length ofstabilized acrylic fibrous material in a concentration of about 200 to2,000 parts per million prior to heating said fibrous material aboveabout 500* C. in said heating zone.